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Possible 4-20mA to 1-5V conversion problem

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Hi all,

I'm working in a project where I have do read from one 4-20mA output sensor (LU-20 - Flowline LU20 EchoTouch™ Two-Wire Ultrasonic Level Transmitter) by the MCP3208 ADC (using PIC18F2620 and others stuffs also).

I built the signal conditioning circuit by means of LM358 OPAMP with differential inputs (see input.jpg) and one 250 Ohm resistor (to get 0-5V from 4-20mA).

During some tests I applied 0-5V through the 250 Ohm circuit input resistor and everything worked fine and the ADC outputs were as they were supposed to be, but when I put the circuit input (its 250 Ohm resistor) in the 4-20mA sensor closed loop, I noticed that the ADC outputs began to fluctuate around the values they should be.

Even using a single power supply (sensor and circuit in the same supply with same ground for both) or using two power supplies (one for the sensor and another one for the circuit), the ADC output fluctuation still exist.

I have already spent a good amount of time trying to solve this problem with no success.

Does anyone have any idea?
Any help would be appreciated.


Thank you,
Rodrigo
 

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I would assume, this being an ultrasonic ranging type device, that the current output would wobble around a bit. You might want to turn on echo averaging in the LU20. Also, since the device is powered by the current loop, you will want to put a low pass filter ahead of the ADC to filter out any noise induced in the loop.
 
kchriste said:
I would assume, this being an ultrasonic ranging type device, that the current output would wobble around a bit. You might want to turn on echo averaging in the LU20. Also, since the device is powered by the current loop, you will want to put a low pass filter ahead of the ADC to filter out any noise induced in the loop.

Hi kchriste,


First of all, thank you for your reply.

I already tested the same circuit using the echo average and with an lp filter. I got the same problem. I also already tested the circuit using a pressure transmitter and the problem remains, even when the sensor output are 4mA. So, I think it may be an issue of my circuit. Anyway, I'm going to test it again following yours instructions.


Thank you,
Rodrigo
 
Here's a thought: Is that difference amp configuration ok for single-supply use? I suspect it isn't, though I don't know if that'd cause the problems you're having.
 
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hi Rodrigo,
Can you post a diagram showing how the sensor, powersupply ground lines are connected to you opa?
It sounds like a ground loop problem at first reading.
Whats the magnitude and frequency of the reported fluctuations?

I assume you have used closely matched resistors for CMMR in your opa.


With the system connected, what voltages do you have at each input pin [either end of the 250R] with respect to 0V
 
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1 to 5 v

rprodrigues said:
Hi all,

I built the signal conditioning circuit by means of LM358 OPAMP with differential inputs (see input.jpg) and one 250 Ohm resistor (to get 0-5V from 4-20mA).

Rodrigo

Hi Rodrigo,

Using 250:eek:hm: resistor with 4-20 mA will be as follows:

250*4 = 1000 mv = 1 volt
250*20 = 5000 mv = 5 volts

so, your range is 1 to 5 volts ( only 4 volts of range with 1 volt elevation )
 
Connections diagram

Tomble, ericgibbs and aljamri, thank you for yours repplies.

Aljamri, it was my fault. I wrote it wrong. I should had written "(to get 1-5V from 4-20mA)". Thank you!

Tomble, I think it can be really the problem, so, as ericgibbs asked, I put the connections diagram so we can check it.

ericgibbs, here they are: two_supplies.png and single_supply.png.

I've also realized some strange behavior:

1 - When I use single supply connection, when I measure the Voltage (circuit GND as measuring reference) at the input pin of the ADC, I get always an apparently constant correct value (as it should be after passing through the OPAMP configuration), even when ADC output is fluctuating.
For example: with 4mA from the sensor, 1V is gotten through the 250 Ohm resistor and also at the ADC input pin;

2 - With two power supplies, the 4-20mA sensor power supply of 12-13V, and the current of 4mA through closed loop, I still get 1V over the 250 Ohm resistor, but I get about 1.64V at the ACD input (reference at circuit GND);

3 - When I use a power supply of 24V for the 4-20mA sensor, for 4mA in the closed loop I still get 1V over the 250 Ohm resistor, but now I get about 2.25V over the ACD input (reference at circuit GND).

4 - The values gotten in cases 2 e 3 seems to be constant at the multimeter (voltage meter), by the ADC outputs fluctuates.

I hope it may give us some direction.

If I convert the 4-20mA of the closed loop with another circuit and them use an linear opto between the closed loop circuit and the circuit input, it would help?

Thank you again!!!!!
 

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ericgibbs said:
hi Rodrigo,
Can you post a diagram showing how the sensor, powersupply ground lines are connected to you opa?
It sounds like a ground loop problem at first reading.
Whats the magnitude and frequency of the reported fluctuations?

I assume you have used closely matched resistors for CMMR in your opa.


With the system connected, what voltages do you have at each input pin [either end of the 250R] with respect to 0V

Hi ericgibbs,

I put the diagram in my last post. I hope they may help us.

Thank you,
Rodrigo
 
hi Rodrigo

Disconnect the output of the LM358 from the ADC pin#1.

Using an oscilloscope measure the output signal from the LM358 pin#1 to see that it is 'clean' ie: free from electrical noise.
You may see a mains supply related signal at 50/60HZ

Also measure the Vdc levels at the 'top' end of the 250R, with 4mA you should get +1V and also +1V at the output of the LM358.

From what you are describing you have a ground loop noise problem.

Does the two core cable from the 4-20mA transmitter have a screen, if so where is it connected?

Is the body of the transmitter sensor or any lead at the sensor end grounded?
 
ericgibbs said:
hi Rodrigo

Disconnect the output of the LM358 from the ADC pin#1.

Using an oscilloscope measure the output signal from the LM358 pin#1 to see that it is 'clean' ie: free from electrical noise.
You may see a mains supply related signal at 50/60HZ

Also measure the Vdc levels at the 'top' end of the 250R, with 4mA you should get +1V and also +1V at the output of the LM358.

From what you are describing you have a ground loop noise problem.

Does the two core cable from the 4-20mA transmitter have a screen, if so where is it connected?

Is the body of the transmitter sensor or any lead at the sensor end grounded?


Hi Eric,


Is the cable screen that woven wire shield around that acts as an electrical shield? If yes, then the cable in use doesn't have it.

The only sensor connection is the 2 wire power supply that acts also as the 4-20mA transmitter line. Its body or even any other of its parts aren't grounded.

I'm going use the oscilloscope to check what you said and after that I will told you what I will have found.


Thank you again!

Rodrigo.
 
Possible 4-20mA to 1-5V conversion problem (solved)

ericgibbs said:
hi Rodrigo

Disconnect the output of the LM358 from the ADC pin#1.

Using an oscilloscope measure the output signal from the LM358 pin#1 to see that it is 'clean' ie: free from electrical noise.
You may see a mains supply related signal at 50/60HZ

Also measure the Vdc levels at the 'top' end of the 250R, with 4mA you should get +1V and also +1V at the output of the LM358.

From what you are describing you have a ground loop noise problem.

Does the two core cable from the 4-20mA transmitter have a screen, if so where is it connected?

Is the body of the transmitter sensor or any lead at the sensor end grounded?


Hi ericgibbs,


First of all, thank you for your so valuable hints/tips. You were right.

There was really a 60Hz wave there. So, the problem was solved when I used the one-power-supply connection.

I still can't realize what happen when I do a two-power-supplies connection. Should I get no problem when I do a two-power-supplies connection?

Thank you very much !

Best regards,
Rodrigo
 
rprodrigues said:
Hi ericgibbs,


First of all, thank you for your so valuable hints/tips. You were right.

There was really a 60Hz wave there. So, the problem was solved when I used the one-power-supply connection.

I still can't realize what happen when I do a two-power-supplies connection. Should I get no problem when I do a two-power-supplies connection?

Connection of the PSU and signal lines are very important, try to use a 'star' ground/common connection for the PSU's and signals.
If you go and search www.analog.com for grounding techniques you will find their application notes useful
.

It could be one of your power supplies [PSU] isnt 'ground' free on the 0V output

Thank you very much !

Best regards,
Rodrigo

Hi,
Hope this helps.

Good luck with the project.
 
project

hello Sir,
i m nitin from bikaner.
and i just i want a circut diagram for
"design a signal conditioning circuit for conversion of 0 to 1 volt to 4 to 20 mA"
so plz send me this circut diagram.
 
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Why a no-gain differential amp? Why not do a voltage follower?

Also, how did you guess 50/60Hz interference?
 
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can someone draw me a circuit so i can convert the output from a Pressure transmitter (4-20mA) into 1-5v

basically 4-20mA to 1-5v conversion circuit.

any help would be greatly appreciated due to my very basic knowledge of electronics

my email address is smo_uk (at) hotmail.com
 
can someone draw me a circuit so i can convert the output from a Pressure transmitter (4-20mA) into 1-5v

basically 4-20mA to 1-5v conversion circuit.

any help would be greatly appreciated due to my very basic knowledge of electronics

my email address is smo_uk (at) hotmail.com

Simple. Use a 250 Ω resistor. Tolerance of 0.1% if you can.
 
but how do i do that.

would it be possible for u to draw me the circuit diagram please?

it would be greatly appreciated

thanks
 
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